Engineers use compression (also called source coding or source encoding) to reduce the bit rate of digital video. Compression decreases the cost of storing and transmitting video information by converting the information into a lower bit rate form. Decompression (also called decoding) reconstructs a version of the original information from the compressed form. A “codec” is an encoder/decoder system.
Over the last two decades, various video codec standards have been adopted, including the ITU-T H.261, H.262 (MPEG-2 or ISO/IEC 13818-2), H.263, H.264 (MPEG-4 AVC or ISO/IEC 14496-10), and H.265 standards and the MPEG-1 (ISO/IEC 11172-2), MPEG-4 Visual (ISO/IEC 14496-2) and SMPTE 421M standards. A video codec standard typically defines options for the syntax of an encoded video bitstream, detailing parameters in the bitstream when particular features are used in encoding and decoding. In many cases, a video codec standard also provides details about the decoding operations a decoder should perform to achieve conformant results in decoding. Aside from codec standards, various proprietary codec formats such as VP8 and VP9 define options for the syntax of an encoded video bitstream and corresponding decoding operations.
A video source such as a camera, animation output, screen capture module, etc. typically provides video that is converted to a format such as a YUV 4:4:4 chroma sampling format. In a YUV format, sample values of a luma (Y) component represent brightness or intensity values, and sample values of chroma (U, V) components represent chroma values. In a YUV 4:4:4 format, chroma information is represented at the same spatial resolution as luma information.
Many commercially available video encoders and decoders support only a YUV 4:2:0 chroma sampling format. A YUV 4:2:0 format is a format that sub-samples chroma information compared to a YUV 4:4:4 format, so that chroma resolution is half that of luma resolution both horizontally and vertically. As a design principle, the decision to use a YUV 4:2:0 format for encoding/decoding is premised on the understanding that, for most use cases such as encoding/decoding of natural camera-captured video content, viewers do not ordinarily notice many visual differences between video encoded/decoded in a YUV 4:2:0 format and video encoded/decoded in a YUV 4:4:4 format. The compression advantages for the YUV 4:2:0 format, which has fewer sample values per frame, are therefore compelling. There are some use cases, however, for which video has richer color information and higher color fidelity may be justified. In such use cases, the differences between YUV 4:4:4 and YUV 4:2:0 chroma sampling formats are more easily perceived by viewers. For example, for encoding/decoding of computer screen text content, animated video content with artificial hard-edged boundaries, or certain features of video content more generally (such as scrolling titles and hard-edged graphics, or video with information concentrated in chroma channels), a 4:4:4 format may be preferable to a 4:2:0 format. Although screen capture codecs that support encoding and decoding in a 4:4:4 format are available, the lack of widespread support for codecs supporting 4:4:4 formats (especially with respect to hardware codec implementations) is a hindrance for these use cases.